Written By: Steven Robert Cypherd
Breadboard kit (1)
Digital multimeter (1)
LEDs are everywhere and in every project you see anywhere. They show that your
microprocessor unit is alive and running the program you wrote. LEDs are tough little lights
that light up everything including you. They provide instant gratification. Here is my simple
tester. There are lots of kits to show you electronics and soldering and a lot of them use
LEDs. Even high-end electronics use LEDs that nobody ever sees to tell a technician that a
circuit is working.
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Step 1 — LED Testing
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Testin an LED
C 3 Jf™ lts S tep 2 or 1&2
Resistors: 220, 330,
460, 820, 1000
Disconnect Pot for
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• Build the circuit as shown in the
picture. Always wire the pot so that
clockwise rotation increases the
voltage or lessens the resistance.
• Wiring connections: Ground to
LED-; Pot-2 to LED+; Pot-3 to
Volts+; DMM to DC Volts; DMM-
to ground; DMM+ to LED+.
• Turn the pot counter-clockwise all
of the way. Connect power.
• LEDs are Current Devices and they
have three states. 1 : Lit but dim.
Low light voltage. Saves batteries.
2: Lit and steady. Normal
brightness. 3: Too bright. You are
over-driving them and they will
burn out sooner. Lastly, state 4:
Too bright and dimming. You have
just killed an LED.
• Rotate the pot clockwise until the
LED begins to light. You can note
this as the lowest voltage that will
light the LED. Note the voltage on
the DVM. Keep the voltage to less
than 2.2 volts for most LEDs.
• Keep watching the LED as you are
turning the pot clockwise until the
LED brightness is steady. This is
your LED's operating voltage. Turn
the pot counter-clockwise to the
lowest point where the brightness
of the LED is steady. This voltage
will make the LED last longer.
• Turn the pot too far and the LED
will start to dim. Just turn the pot
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counter-clockwise until the LED
brightness is steady. Keep the
voltage at about 2 volts for
maximum brightness for most
• Some LEDs have a circuit in them
that controls voltage and current to
the LED. They will not light until
they have reached their normal
operating voltage. They come on
and stay at their normal brightness
over a wide range of voltage and
current. Usually this is between 4
and 12 volts.
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• Batteries can produce a lot of
current, so test LEDs as shown
and use the largest resistor you
can for the best brightness. Normal
LEDs run about 7 - 30- milli-
amperes (mA) each. Some high-
brightness LEDs and LEDs with
chips in them can draw 200 mA
each. Above 5 volts I usually use a
460 - 1 000 (1 K) ohm resistor for
normal LEDs and 22 - 200 ohms
for high-brightness LEDs or LEDs
with chips. The idea is to protect
your LEDs for a longer life.
• Limiting the current through each
LED with a resistor on the positive
lead will give your projects a longer
life. It does not save your batteries.
Lowering the brightness of your
LEDs can save your batteries a
• Current testing: Testing the current
drawn by the LED is a better test
for a current-driven LED. Replace
the red wire from Pot-2 to LED+
with a second DMM or use the
same DMM as shown.
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• Run Step 1 and note how the
voltage changes slowly, but when
you hit the operating voltage of the
LED the current will jump up to the
normal current of the LED. As you
increase the current the brightness
of the LED will start to dim. Turn
the pot counter-clockwise until the
current and the brightness is
steady. Always keep the current on
the low side and your LEDs will last
• Repeat for more LEDs.
• Finding the resistor for the LED at
• Disconnect Power
• Set DMM to Ohms. Disconnect the
• Measure across Pot-2 and Pot-3 to
get the resistor you will need for
that LED at that voltage. Find the
closest higher resistor for your
LED. Keeping the current lower will
keep the LED healthy. Remember,
the brighter the LED the shorter its
life will be.
• Reconnect the pot.
• Turn the pot full counter-clockwise.
Repeat the above steps for more
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LEDs light the world up and they are easy and fun.
This document was last generated on 2012-11-03 12:30:46 AM.
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